The present invention relates generally to optical devices and, more particularly, to optoelectronic devices based on electron tunneling.
The increasing speed of optical communications is fueling the race to achieve ever faster optical communications devices for transmitting, modulating and detecting electromagnetic signals. Terahertz speeds are expected in the near future, and optical communication devices that can operate at such high speeds are in great demand.
A possible approach to achieving high speed optoelectronic devices for use as optical communication devices is electron tunneling. Electron tunneling-based devices, such as metal-insulator-metal (M-I-M) devices for use as infrared and far-infrared detectors and frequency mixers have been explored in the past (see, for example, S. M. Faris, et al., “Detection of optical and infrared radiation with DC-biased electron-tunneling metal-barrier-metal diodes,” IEEE Journal of Quantum Electronics, vol. QE-9, no. 7 (1973); L. O. Hocker, et al., “Frequency mixing in the infrared and far-infrared using a metal-to-metal point contact diode,” Applied Physics Letters, vol. 12, no. 12 (1968); and C. Fumeaux, et al., “Nanometer thin-film Ni—NiO—Ni diodes for detection and mixing of 30 THz radiation,” Infrared Physics and Technology, 39 (1998)). Such M-I-M devices generally operate on the basis of electron rectification and current production due to incident electromagnetic energy and resulting electron tunneling effects. M-I-M devices can normally be used to rectify extremely high frequencies, extending into the optical frequency range.
In addition to high frequency rectification, it is also desirable to achieve high degrees of asymmetry and nonlinearity in the current-versus-voltage (I-V) curve in electron tunneling devices. The differential resistance of the device, which corresponds to the sensitivity of the device to incoming electromagnetic energy, is directly related to the nonlinearity of the I-V curve. However, prior art M-I-M devices generally exhibit low degrees of asymmetry and nonlinearity in the electron transport such that the efficiency of such devices is limited. A high degree of nonlinearity improves the quantum efficiency of the electron tunneling device, which is number of electrons collected for each photon incident on the M-I-M device. High quantum efficiency is crucial for efficient operation and high sensitivity of the M-I-M diode in all optoelectronic applications. For purposes of this application, a diode is defined as a two-terminal device. A high degree of nonlinearity offers specific advantages in certain applications. For example, in optical mixing, second order derivatives of the current-voltage relationship determine the magnitude of the signal produced in frequency down-conversion. A higher degree of asymmetry in the I-V curve between positive values of V (forward bias voltage) and negative values of V (reverse bias voltage) results in better rectification performance of the device. A high degree of asymmetry is required, for example, to achieve efficient large signal rectification such as in the detection of high intensity incident fields. In this high intensity field regime, the electron tunneling device functions as a switch, and it is desirable to attain a low resistance value in one polarity of voltage and a high resistance value in the opposite polarity of voltage is desired. Alternatively, with low field intensities and large photon energies, the incident field sweeps a larger portion of the electron tunneling device dark I-V curve and, consequently, the high asymmetry translates into high responsivity and as well as high quantum efficiency and sensitivity in electromagnetic radiation detection.
The fabrication of the combinations of alternate layers of metals and insulators in M-I-M-based devices, in comparison to semiconductor materials, is advantageous due to ease of deposition of materials relative to semiconductor fabrication. It has been suggested that the recent trend of decreasing the size of electronic devices to achieve high speed switching will ultimately make semiconductor-based devices impractical due to fluctuation of carrier concentration, which occurs when semiconductor devices are reduced to mesoscopic regimes (see, for example, Suemasu, et al., “Metal (CoSi2)/Insulator(CaF2) resonant tunneling diode,” Japanese Journal of Applied Physics, vol. 33 (1994), hereafter Suemasu). Moreover, semiconductor devices are generally single bandgap energy devices. This characteristic of semiconductor devices means that, in detection applications, no current is produced when a photon having energy less than the bandgap energy is incident on the semiconductor device. In other words, the response of the semiconductor device is limited by the bandgap energy. When a semiconductor diode is used to rectify high frequency oscillations, the semiconductor material limits the frequency response of the diode because the charge carriers must be transported through a band, in which concentration are limited in comparison to a metal.
Existing electron tunneling devices based on metal-oxide combinations are generally fabricated by forming a metal layer, exposing the metal layer for a certain amount of time such that the native oxide of the metal is formed, then repeating the process as desired. Photolithography techniques may also be used to achieve desired shapes and patterns in the metals and insulators. For example, Suemasu describes a metal (CoSi2)/insulator(CaF2) resonant tunneling diode with a configuration M-I-M-I-M-I-M triple-barrier structure for use as long wavelength (far-infrared and milliwave) detectors and emitters. However, the M-I-M-I-M-I-M device of Suemasu is much more complicated than the simple M-I-M devices, and must be fabricated using a complex epitaxial growth procedure using exotic materials. In fact, Suemasu chooses to use the triple-barrier structure rather than a slightly simpler double-barrier structure for apparently better performance results in the electron tunneling process. Therefore, although the M-I-M-I-M-I-M device of Suemasu achieves much higher degrees of asymmetry and nonlinearity in the I-V curve than the M-I-M devices, the performance gains come at the cost of the simplicity in design and fabrication.
An alternative approach is the use of a combination of a metal and a semiconductor in a metal-insulator-semiconductor (MIS) configuration (see, for example, T. Yamada, et al., “Semiconductor Device Using MIS Capacitor,” U.S. Pat. No. 5,018,000, issued 21 May 1991). The drawback to currently available MIS devices is also the limited efficiency due to asymmetry and nonlinearity limitations. MIS devices cannot operate at as high frequencies as M-I-M devices because the concentration of electron states in the semiconductor is lower than that from a metal.
At this time, infrared detectors, for example, capable of receiving electromagnetic signal at terahertz rates, at room temperature, are not readily available, to the Applicants' knowledge. Temperature-controlled alternatives, such as narrow bandgap semiconductor detectors, and bolometers, exist on the market, but the extra considerations associated with the temperature control mechanism make such devices expensive and bulky. Prior art M-I-M detectors are capable of detecting infrared radiation without cooling, but these prior art detectors are not sensitive enough for practical applications.
As will be seen hereinafter, the present invention provides a significant improvement over the prior art as discussed above by virtue of its ability to provide the increased performance while, at the same time, having significant advantages in its manufacturability.